1. Field of the Invention
The present invention is generally related to nuclear reactors and, more particularly to nuclear propulsion reactors.
2. General Background
Nuclear rocket engines present an attractive choice for providing propulsive thrust because they have a much higher theoretical specific impulse than chemical rocket engines. Specific impulse is a measure of the fuel efficiency of a rocket engine. The very best chemical rocket engines have a specific impulse of approximately 450 seconds, meaning that they consume one pound of propellant every second for every 450 pounds of thrust they produce. The specific impulse of chemical rocket engines is limited to the low 100's of seconds range by the limited amount of chemical energy stored in the propellant. Since nuclear rocket engines utilize far more energetic nuclear processes to heat the propellant, they are not limited by the same considerations as chemical rocket engines. The theoretical specific impulse of various nuclear rocket engine concepts ranges from a low of approximately 750 seconds to values greater than 10,000 seconds.
Nuclear rocket concepts can generally be grouped into the categories of nuclear thermal rockets and nuclear electric rockets. Nuclear thermal rockets utilize the heat generated in the nuclear reactor core to directly heat a propellant that is then expelled through a nozzle to produce thrust. Solid core nuclear thermal rockets, such as the ROVER-NERVA rockets and their ENABLER-I and ENABLER-II derivative designs, are particularly prone to high engine weight and low thrust-to-weight ratios due to their use of a high-temperature carbon-based reactor core. Carbon is a poor neutron moderator. This results in an epithermal neutron spectrum in the core and large critical masses. Materials having moderator characteristics superior to carbon generally have a much lower allowable operating temperature and can not tolerate the high temperatures present in a nuclear rocket engine. These characteristics make it particularly difficult to design a small nuclear rocket utilizing ROVER-NERVA technology. Patent applications directed to nuclear rockets which applicant is aware of include the following.
Copending U.S. application assigned Ser. No. 07/895,054 discloses a two stage nuclear propulsion reactor. An annular first core radially encompasses a second core to provide two stage heating of the propellant as it is directed through the reactor.
Copending U.S. application assigned Ser. No. 07/822,391 discloses a thermionic reactor that provides electrical and propulsion power. Thermionic heat pipe modules located within the reactor vessel convert heat into useful electrical energy and serve to remove waste heat. Coolant/propellant travels through the reactor vessel and is directed to a propellant nozzle for producing propulsive thrust.
The known art is not directed to the problems associated with the utilization of a high performance moderator in combination with a high-temperature carbon based reactor core.